The present invention relates to a method and apparatus for measuring the resonance of noble gas nuclei in a magnetic field, such as may be useful for constructing sensitive gyroscopes or magnetometers, and in particular to a resonator that reduces the effect of the magnetic fields of alkali atoms used to polarize and/or detect the noble gas spins.
The nuclei of isotopes of noble gases such as xenon have a net spin which possesses a magnetic moment. These noble gases tend to be insensitive to environmental perturbations, except those which couple to the nuclear magnetic moments. Accordingly, if the nuclei of such atoms can be polarized and stimulated into precession, the frequency of precession can be used to precisely measure a magnetic field free from other influences. In this way, a precision magnetometer may be constructed.
Alternatively, it will be understood that if the magnetic field is known and constant, deviations in the resulting frequency of precession, measured in the reference frame rotationally stationary with respect to the measurement of the precession, will indicate rotation of the observer about the axis of precession. In this way, a precision gyroscope may be constructed.
The qualities of noble gas that make them relatively insensitive to environmental conditions other than the magnetic field may conversely make it difficult to enforce a net polarization of these atoms in the small magnetic fields typically at issue. Accordingly, spin-polarization of the noble gas may be accomplished in a two-step process in which alkali atoms such as rubidium are first spin-polarized by pumping them with a circularly polarized pump beam from a laser. The spin-polarization of the alkali atoms is then transferred to the noble gas isotopes by collision.
In the construction of a gyroscope, the magnetic fields must be carefully controlled. Generally, the Earth's magnetic field has some variability, the influence of which may be reduced by magnetic shielding. In addition, the magnetic field produced by the magnetic moment of the alkali atoms adds a poorly known local component to the magnetic field. Both of these unknowns may be reduced by the use of two isotopes (for example 129-xenon and 131-xenon) as follows:
Generally the two xenon isotopes will have different gyromagnetic constants γ1 and γ2 such that:ω1=γ1(B0+BA1)±Ω  (1)ω2=γ2(B0+BA2)±Ω  (2)
where ω is the measured precessional frequency, B0 is the magnetic field (a combination of all magnetic field influences including that of the alkali atoms), BA1 and BA2 are the magnetic field produced by the alkali atoms, and Ω is the angular rotational rate of the system to be determined. The sign of the gyromagnetic constants determines whether the rotation adds to or subtracts from the resonance frequency. For xenon, BA1=BA2, so that measurements of the two precession frequencies ω1 and ω2 allows the system of two independent equations to be solved for Ω reducing the influence of the unknown value of B0 and BA to the limits of the measurement precision.
The use of xenon atoms has a drawback insofar as one xenon isotope (Xe 131) is subject to quadrupolar electric interaction causing it to be sensitive to electrical field gradients, and thus adds an extra term to one precession frequency violating the assumption that the noble gases are generally insensitive to environmental conditions other than magnetic field. The quadrupolar interaction can be eliminated through the use of a different noble gas such as a helium isotope, for example, 3-helium, which has no electrical quadrupole moment. Unfortunately, the helium atoms have a different value for BA as compared to xenon, so the 3-helium/xenon mixture is far more influenced by the magnetic field of the alkali atoms used to promote spin polarization than the xenon isotopic mixture. Imperfect correction for the magnetic field of the alkali atoms imposes an error on the precision of the measurement of the precession of the helium or xenon atoms, largely negating any improvement obtained by replacing one xenon isotope with helium.